Means and method for signaling



Deco 28 1926.

W. E. BEATTY MEANS AND METHOD FOR SIGNALING Filed May 21, 1921SUPP/755558 o l'oIOOO .S UPPFTE SSE S o ro mun SUPPRESS v 300 man [500F0 To To 400 I200 1600 UPPRE $2.

300 I000 I509 1"0 f'o To 400 I200 I600 SUPP/9E 555 I000 To 20060SUPERESSES 55 5 OTnl SUPPHE SUPPHESSES 1000 to 20000 Patented Dec. 2%,19260 WILLIAM E. rnnrrr, or" BAYSIIDE, nnw

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YORK, ASSIGNOR T0 WESTERN ELECTRIG COMPANY, INCORPGRATED, 0F YORK, N.Y., A CORPORATION OF NEW YORK. 4

MEANS AND METHOD FOR SIGNALING.

Application filed May 21,

This invention relates to means and methods for signaling and moreparticularly it relates to secret signaling.

One of the objects of this invention is to provide a signaling methodand system by means of which signals, such as telephone signals, may betransmitted to a distant station without any danger of the'signals beingintelligently received by persons not equipped with the specialsignaling apparatus of this invention.

Another object of this invention is to provide a secret signaling systemwhich does not require any means for synchronizing the apparatus at thetransmitting and receiving stations, which is a necessary element ofsecrecy systems heretofore proposed.

This invention broadly contemplates transmitting to an outgoing line, asignal of a single frequency or a plurality of frequencies, impressingsimultaneously on the outgoing line disturbing currents of otherfrequencies and providing a receiving device which is responsive to thesignaling frequencies but unresponsive to the isturbing frequencies.

The preferred embodiment of this invention by means of which the aboveobjects are obtained comprises, when employed for the transmission ofspeech, a wave filter at the transmitting station located between thetelephone transmitter and the outgoing line and so designed that thefilter suppresses certain of the speech frequencies or a band offrequencies within the speech range while allowing the remaining speechfrequencies to be transmitted to the outgoing line. Noise currents offrequencies-corresponding to the suppressed speech frequencies are alsoimpressed on the line so that the signaling line at any particularinstant contains both speech frequencies and noise frequencies, thenoise frequencies being of such an intensity as to render the signalswholly unintelligible. At the receiving station a wave filter similar tothat at the transmitting station is provided for suppressing the noisefrequencies while permitting the signaling frequencies to pass on to asuitable receiving device.

This invention will be better understood by reference to the followingdetailed description taken in connection with the accompanying drawingsin which in Fig. 1,

1921. Serial No. 471,331. 7

a wave filter is employed at both the transmitting and receivingstations for suppressing a certain portion of the speechfrequenciesfmeans being provided for impressing on the signaling linenoise frequencies corresponding to the suppressed frequencies of thespeech signal; Fig. 2 represents a modification of the system of Fig. 1,in which a plurality of filters are employed at both the transmittingand receiving stations for suppressing bands of frequencies throughoutthe. speech range, means being provided for impressing on the signalingline, noise currents of frequencies corresponding to the suppressedbands, and Fig. 3 represents a modification of the arrangement of Fig. 1arranged for two-way opera tion.

The secret signaling system of Fig. 1 illustrates a form particularlyadapted for the secret transmission of telephone signals. Thetransmitting station of the system comprises a telephone transmitter 10which is connected to the signal line 11, 12 through a wave filter 13.The impedance elements comprising wave filter 13 should be so ar rangedand should have such values that the filter serves to completelysuppress an appreciable portion of the speech frequencies generated bythe transmitter 10 whereby the transmission line 11, 12 containscurrents of only a ortion of the various frequencies generated by source10. For example, wave filter '13 may have its values so chosen that itwill allow frequencies between 1000 cycles and 3000 cycles to betransmitted freely while suppressing frequencies outside this range. Thearrangement and the values the wave filter should have to produce thisresult are described in the Campbell U. S. Patent No. 1,227,113 of May22, 1917, and, for the purpose of this invention, it is not believednecessary to embody in this specification, a complete description of thedesi formulae: for wave filters as is outlined in the above patent.

Inasmuch as telephone currents are still intelligible in a telephonereceiver even after.

they have been distorted by the suppression of an appreciable portion ofthe frequencies representing speech, it follows that the suppression ofa portion of the speech frequencies by wave filter 13, does not render,the

speech from source 10 unintelligible although, of course, the quality ofthe speech will be considerably modified thereby.

In accordance with this invention, the speech currents in line 11, 12are rendered wholly unintelligible by impressing on the signal line adisturbing noise current corresponding to a range of frequenciessuppressed y wave filter 13. For example, in the case where filter 13suppresses all frequencies below 1000 cycles, the source of noisecurrents 14 may be so designed as to impress on the signal line 11, 12currents of frequencies ranging from 0 to 1000 cycles per second. Thesenoise currents, of course, should have such an intensity as to preventone, by an ordinary receiving device such as a telephone receiver, frombeing able to distinguish between the noise currents and the speechcurrents in the signal line, and therefore it will generally be foundnecessary to make the intensity of the noise currents at least as greatas the intensity of the speech currents. One form the source of noisecurrents 14 may have comprises a vacuum tube oscillation generator 15 ofthe type now well known in the art in which the frequency generateddepends upon the values of the inductance windings 16 and 17, andcapacities 18 and 19. Inasmuch as it is preferable that source 14generate frequencies continuously varying over a portion of the range offrequencies suppressed by wave filter 13, one of the condensers 18 forexample, may be made of such a type as to permit a motor 20 tocontinuously vary the capacity of condenser 18 in such a manner thatsource 14 through transformer 21 delivers to the signal line currentsvarying, for example, between 0 and 1000 cycles in the case where such arange is suppressed by filter 13.

It is to be understood, of course, that this invention is not limited tothe particular source of noise currents just described since variousmeans, widely different from that described above, may be employedequally well to render the speech frequencies in line 11, 12unintelligible.

The receiving station ofthe system of Fig. 1 comprises a wave filter 22which preferably should be of the same type and design as wave filter 13so that filter 22 transmits, in the assumed case, frequencies only inthe range between 1000 and 3000 cycles per second. Since the noisecurrents impressed on line 11, 12 are outside of this transmission band,it follows that Wave filter 22 will serve to suppress the noise currentsand will allow only the speech currents to be impressed by a transformer23 upon asuitable receiving device such as a telephone receiver 24.

From the above description, it is evident that any one listening in onthe signal line 11, 12 will not be able to decipher the distortedsignals unless he is provided with a wave filter of the same design asthat employed at the true transmitting and receiving stations.

()ne of the chief advantages of the above system is that nosynchronizing apparatus is required to synchronize the various parts ofthe receiving station with corresponding parts at the transmittingstation as is generally required in secret signaling systems heretoforeproposed. The system of this invention has therefore been described incertain of the appended claims as a system in which the decipheringmeans at the receiving station acts independently of time, that is,independently of the time the distorting means begins to operate andindependently of the rate of its operation.

As shown in Fig. 1, it is preferable that the source of noise currentsbe located at the transmitting station so that as the noise cur rentsand the telephone currents are transmitted along the line conductor 11,12 both will be attenuated to the same degree and the predominance ofthe noise currents over the telephone currents will be maintained overthe entire length of the signaling c011- ductor 11, 12. It is obvious,however, that if desired the source of noise currents 14 may beassociated with the signaling conductor ata point other than at thetransmitting station. 7

Fig. 2 illustrates a secret telephone system 1n which a plurality offilters are employed at boththe transmitting and receiving stations forsuppressing bands of frequencies throughout the speech range, meansbeing provided for impressing on the signaling line, noise currents offrequencies corresponding to the suppressed bands. The transmittingstation of this system comprises a plurality of wave filters 26, 27 and28 located between the signaling line 29, 30 and the source of telephonecurrents 31. For illustrative purposes, it may be assumed that filter 26is so designed that it will suppress currents of frequencies lyingbetween 300 and 400 cycles per second, filter 27 being designed tosuppress currents of frequencies between 1000 and 1200 cycles and filter28 being designed to suppress currents of frequencies between 1500 and1800 cycles. In order to provide noise currents for the line 29, 30having frequencies corresponding to the suppressed bands, a plurality ofsources of noise currents 32, 33 and 34 are provided, source 32 beingadapted in the case assumed, to supply currents of frequenciescontinuously varying between 300 and 400 cycles, source 33 supplyingcurrents of frequencies between 1000 and 1200 cycles and source 34supplying currents of frequencies between 1500 and 1800 cycles. Thesethree sources 32, 33 and 34 may be of the type, for example, shown inFig. 1. The receiving station for the signals from source 31 compriseseraoeo filters 35, 36 and 37 corresponding to filters 26, 27 and 28 ofthe transmitting station. The insertion of these three filters betweenthe telephone receiver 38 and the signaling conductor serves to preventthenoise currents from being impressed on the receiver while allowingtelephone currents unsuppressed by filters 26, 27 and 28 to be receivedthereby. It is to be understood, of course, that any desired number ofbands of s )eech frequencies may be suppressed by suitable wave filtersand that any desired number of sources of noise currents may be employedfor generating currents of the suppressed frequencies.

Fig. 3 illustrates an arrangement whereby the system of Fig. 1 may beemployed for two-way transmission between two stations. Wave filter 40at one of the stations may be designed to suppress frequencies between 0and 1000 cycles thereby preventing currents in this range from beingimpressed on the signaling line 41, 42 by the telephone transmitter 43.Noise currents corresponding to the suppressed band are impressed online conductor 41, 42 by the source of noise currents 39 through atransformer 44. At the other station, a wave filter 45 corresponding towave filter 40 is inserted between the telephone receiver 46 and thesignaling line, to allow only currents from the telephone transmitter 43to be impressed on the re ceiver 46. In order to provide fortransmission in the reverse direction, wave filter 47 which is locatedbetween the telephone trans mitter 48 and the signaling conductor shouldbe of such a type as to suppress a band of frequencies which is freelytransmitted by the wave filter 45. In the case assumed, wave filter 47should therefore be designed to suppress currents between 1000 and20,000 cycles or the upper limit of frequency of importance in speech.The connections to the signaling line 41, 42 for the telephone receiver49 should be such that the noise currents from source 39 are notimpressed on the telephone receiver. This precaution is necessary sincethe associated wave filter 50 is designed to transmit currents of thefrequencies generated by source 39. One way this result may beaccomplished is to connect one of these leads 51 from the receiver toconductor 41 and to connect the other lead 52 to the midpoint of thesecondary winding of transformer 44. If, now, the impedance relations ofthe system are made such that points 53 and 54 are at equal potentialfor the noise currents, it follows that currents from source 43 willnotbe impressed on receiver 49. The telephone currents, however, fromtransmitter 48 will not be sub ject'to this neutralizing action and willbe freely transmitted by wave filter 50 to the telephone receiver 49.Receiver 49 will not 'freceive any of the transmitted currents from itsassociated transmitter 43 for the reason that Wave filter 50, asspecified above, is so designed as to suppress all of the frequenciestransmitted by wave filter 40. Similarly, receiver 46 at the otherstation will not receive any of the current; generated by source 48 forthe reason that wave filter 45 is designed to suppress all frequenciestransmitted by wave filter 47. It therefore follows that for thetransmission of signals between transmitter 43 and re- .ceiver 46, thesignaling line 41, 42 will contain signaling frequencies from 1000cycles to the limit of audibility, at the same time containing noisecurrents from 0 to 1000 cycles. For transmission in the oppositedirection between transmitter 48 and receiver 49, the signaling line 41,42 will contain signaling currents from 0 to 1000 cycles and will alsocontain noise currents within the same range thereby insuring secrecy insignaling in both directions.

It is to be understood that the forms of this invention described abovemay be variously modified without in any wise departing from the spiritof this invention as defined in the appended claims.

l/Vhat is claimed is:

1. The method of secret signaling over a path extending betweenseparated stations which comprises generating at one station a band ofwave frequencies representing sig nals, transmitting to a second stationwith uniform small attenuation from the generated band a continuous bandof frequencies suficient to enable the reproduction of intelligiblesignals at said second station, suppressing a different band of thefrequenc es of said generated band, impressing on the transmission pathnoise currents of frequencies corresponding to the suppressedfrequencies and embracing a total frequency range comparable with thatof the transmitted signal band to render the transmitted continuousbandof frequencies diflicult of reception, and suppressing the noisecurrents and receiving the signaling frequencies at said second station.

2. A secrecy signaling system comprising a transmitting station, areceiving station, a line extending between said stations, saidtransmitting station comprisng means for generating a wave band of awide frequency range representing signals, a wave filter between saidmeans and saidline, and arranged a to suppress from said Wave band aub-band of frequencies wh le freely transmitting to said line a secondsub-band of frequencies wider than the suppressed sub-band, saidunsuppressed band being sufficient to enable intelligible reproductionof signals, receiving means at saidireceiving station responsive to sa dunsuppre'ssed band of frequencies, a second wave filter between saidreceiving means and said line and arranged to suppress the samefrequency band as suppressed by said first wave filter, and means forimpressing upon said line noise currents having frequenciessubstantially entirely within the frequency range suppressed by saidfilter and embracing a total frequency range comparable with that of thetransmitted signal sub-band.

3. A secrecy signaling system comprising a transmitting station. areceiving station, a signaling path extending between said stations,means at said transmitting station for generating a continuous wave bandof signaling frequencies, a wave filter between said means and said pathand arranged to suppress a sub-band of said continuous wave band whilefreely transmitting from said continuous band, a sub-band of frequencieswider than the suppressed sub-band and sufficient to enable intelligiblereproductionof signals, receiving means at said receiving stationresponsive to the unsuppressed frequencies, means for impressing uponsaid path unintelligible currents of a band of frequencies correspondingto the band sup-v pressed by said filter, the total frequency range ofsaid impressed unintelligible currents being comparable to that of thetransmitted signal band, and means at said receiving station forpreventing said unintelligible currents from being received by saidreceiving means.

4. A secrecy signaling system comprising a transmitting station, areceiving station, a signaling path between said stations, means at saidtransmitting station for generating a continuous frequency band representing signals, means for suppressing a small frequency band from saidcontinuous band while. freely transmitting the major portion of thefrequencies in said continuous band with substantially the same relativeamplitudes as generated by said means, said .major portion beingsufficient to enable intelligible reproduction of signals at saidreceiving station, means for impressing upon said path unintelligiblecurrents of a band of frequencies corresponding to the band suppressedby said second means, said impressed unintelligible currents having atotal frequency range comparable with that of the transmitted signalband, and means at said receiving station for suppressing the samefrequency band as suppressed by said second means.

5. A signaling system comprising a plurality of separated stations, atelephone transmitter and a telephone receiver at each station, aplurality of wave filters of a type arranged to suppress a widefrequency range while freely transmitting another range, a plurality offilters of a second type to suppress a frequency band freely passed bysaid first type, a filter of said first type being located between saidline and the transmitter of said first station; a second filter of thefirst type being located between said receiver of the second station andsaid line. one filter of said second type being located between saidline and the transmitter of said second station, another filter of saidsecond type being located between the receiver of said first station andsaid line, a source of noise currents of a frequency band correspondingto the frequency band passed by said first type of filter, and couplingmeans between the receiver and the transmitter of said first station,said source and said line arranged to prevent the receiver of said firststation from being energized by currents from said source.

6. A secret signaling system comprising a transmission line extendingbetween separated stations, means for impressing on said line at one ofsaid stations for transmission in one direction a signal comprising aband of a plurality of audible frequencies, means for impressing on saidline at another of said stations for transmission in the oppositedirection a signal comprising a band of a plurality of other audiblefrequencies, receiving means at each of said stations, means forimpressing on said line at the transmitting station noise currentscorresponding to one band of said signaling frequencies, and means forkeeping said noise currents out of the local receiving means.

7. In a secret signaling system, a subscribers set comprising atelephone transmitter and a telephone receiver, a transmission lineassociated with said set, means for impressing on said line currents ofcertain frequencies of importance in speech, and for preventing currentsof other frequencies of importance in speech from being impressed onsaid line by said transmitter, means for impressing on said line noisecurrents comprising a continuous band of frequencies corresponding to aband of the suppressed speech frequencies to render the transmittedspeech signals unintelligible, and means for rendering said receiverunresponsive to currents of frequencies unsuppressed from saidtransmitter while permitting said receiver to be responsive to currentsof the frequencies suppressed from said transmitter.

8. The method of secret signaling over a path extending betweenseparated stations which comprises generating at one station a band ofwave frequencies representing signals, suppressing from said band asub-band of said frequencies, selecting from said band a second sub-bandof frequencies wider than said sub-band suppressed, and transmitting thesub-band s0 selected to a. second station, the transmitted frequenciesbeing sufficient to enable reproduction of intelligible signals at saidsecond station, impressing on the path noise currents of frequenciescorresponding to those of the subband suppressed, and embracing a totalfrequency range comparable to that of the transmitted signaling sub-bandto render the unsuppressed part of said generated band unintelligible,and at the distant station suppressing said noise currents whilepermitting the unsuppressed part of said generated band to be received.

9. In a secret telephone system, a transmitting and a receiving station,means at the transmitting station for setting up electrical variationscorresponding to speech, means including a band filter for selectingfrom the variations for transmission to the receiving station at leastone continuous band of frequency components, the total componentsselected for transmission comprising more than one-half the continuousfrequency range from 100 to 2200 cycles per second, means at thetransmitting station for supplying together with the selected componentsnoise currents of frequencies lying in the range of 100 to 2200 cyclesper second but different from those transmitted by said band filter,said noise currents embracing a total frequency range comparable to thatof the transmitted speech band, means including a similar band filter atthe receiving station for separating the speech components from thenoise components, and a telephone receiver actuated by the selectedspeech components.

10. In a secrecy signaling system, a local station comprising atelephone transmitter and a telephone receiver, a second stationcomprising a second telephone transmitter and a second telephonereceiver, a transmission line between said stations, a wave filterbetween said line and said'first transmitter for suppressing a certainportion of the frequencies of importance in speech, a similar filterbetween said second receiver and said line, a Wave filter between saidsecond transmitter and said line for suppressing another portion of therange of frequencies of importance in speech, a Wave filter similar tosaid last mentioned filter located between said first receiver and saidline, means for impressin on said line noise currents comprising a andof frequencies occupying ap proximately the same position in the frequency spectrum as the speech frequencies suppressed by thefirst-mentioned wave filter, and means for preventing said noisecurrents from being impressed on the firstmentioned telephone receiver.

In witness whereof, I hereunto subscribe my name this 19th day of May A.D, 1921.

LIAM E. BEATTY.

